1. Field of the Invention
The present invention relates to adaptive signal equalizers for adaptively equalizing high data rate signals received via long lengths of cable, and in particular, to gain controllers for controlling the signal gain of such adaptive signal equalizers.
2. Description of the Related Art
As part of the process of recovering data which has been transmitted over a long length of cable at a high data rate, equalization of the received data signal is required in order to compensate for the loss and phase dispersion characteristics of the cable. For example, referring to FIG. 1, the signal losses associated with a cable increase with frequency, and such signal losses become greater as the cable length is increased from a virtually zero length L.sub.0 to greater cable lengths L.sub.1, L.sub.2, L.sub.3, . . . . Therefore, higher order frequency components of the data signal become increasingly attenuated as compared to the lower order frequency components. Accordingly, the degree of signal equalization required increases with frequency as well as cable length.
Further, in those applications where the transmission cable lengths may vary, such equalization must be adaptive by being able to adapt to variations in the transfer function of the cable due to variations in the cable length.
Referring to FIG. 2, a conventional adaptive equalizer 20 includes a unity-gain buffer 22, a high-pass filter 24, a mixer 26 and a signal summation stage 28, interconnected as shown. The input signal V.sub.i is processed by both the unity-gain buffer stage 22 and filtered by the high-pass filter 24. The high-pass filtered signal 25 is mixed with a gain control signal a in the mixer 26. The unity-gain buffered signal 23 and gain-controlled, high-pass filtered signal 27 are summed together in the summation circuit 28 to produce the final output signal V.sub.O.
Referring to FIG. 3, it can be seen that by varying the value of the control signal .alpha., the overall gain of the high-pass filter profile can be adjusted, thereby providing for adaptive equalization of the output signal V.sub.O.
While this conventional technique performs reasonably well, a number of disadvantages exist, particularly when more precise equalization control is desired. For example, depending upon a number of operating parameters of the equalization circuit 20, such as variations in processing during manufacturing and variations in operating voltages and temperatures, the gain factor .alpha. may affect the DC biasing of portions of the circuit 20. Further, the output signal V.sub.O may be affected by variations in the DC bias components within the circuit 20. Accordingly, it would be desirable to have a gain-controlled adaptive equalizer in which the output signal is independent of DC bias conditions and in which DC bias conditions are independent of the gain control input.